Capacitive interaction occurs in all electronic circuits. Accordingly, discrete capacitors are included in the circuits to fulfill a variety of roles including frequency filtration, impedance matching and the production of electrical pulses and repetitive signals. Regardless of the complexity of the design, a capacitor can be thought of as two closely spaced conducting plates which may have equal and opposite charges (±Q) residing on them when a voltage (V) is applied. The scalar quantity called capacitance (C) is the ratio of the charge to the applied voltage. When capacitance increases, a significant charge can be stored and the device can be used like a battery.
Though common batteries have a high energy density, they can only deliver a relatively small current since the current must be generated by a chemical reaction occurring within each storage cell. By contrast, capacitors may have a low energy density but can discharge very quickly—a flexibility which is desirable for many applications. Superconducting magnetic energy storage (SMES) is an alternative, but still suffers from a low storage density combined with impractical mass and thermal complexities.
FIGS. 1A-1C show prior art capacitor designs. FIG. 1A shows a capacitor having electrical leads connected to conducting plates or electrodes 110. An air-gap 115-1 is left between electrodes 110 so that when a voltage is applied, a positive charge accumulates on the electrode with a positive bias. This results in an opposite charge on the other electrode and an electric field pointing from left to right in FIG. 1A. Each of the capacitors depicted in FIGS. 1A-1C is symmetric, i.e. possesses the same capacitance regardless of which electrode receives the positive voltage.
In FIG. 1B, the same capacitor has a dielectric material inserted in the space 115-2 between the electrodes 110. The dielectric constant or relative permittivity of the dielectric material allows the amount of charge (the “capacity” or capacitance of the capacitor) stored on each electrode to increase for the same applied voltage. A higher relative permittivity increases the ability of the dielectric to adjust its distribution of charge in response to the applied voltage; a negative charge accumulates near the positive electrode and a positive charge near the negative electrode. A smaller electric field exists between the electrodes if the relative permittivity is higher.
The stored charge can be further increased by using an electric double-layer capacitor (EDLC) design. EDLC's have higher energy density than traditional capacitors and are sometimes referred to as “supercapacitors”. Energy density can be defined as the amount of charge stored per unit volume. However, the storage density of EDLC's (depicted in FIG. 1C) can still be improved upon. Between electrodes 110, a dielectric material 116 surrounds high surface area electrically-conducting granules 117 distributed in the gap 115-3. A dielectric separator 118 is positioned between two regions of the embedded granules 117. The surfaces of granules 117 on the left of separator 118 are positively charged while the granules 117 on the right develop negative surface charging. The effective surface area of the capacitor is increased which allows even more charge to be stored on electrodes 110 for a given voltage.
Despite these advances, further increases in energy storage density of capacitors may improve upon traditional batteries.